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Abstract
The alignment of nematic liquid crystals by rubbed polyimide surfaces has been well-studied and developed. A novel polyimide film which induced a homeotropic alignment of the nematic liquid crystal without rubbing or with weak rubbing strength was presented. However, there was a transition from homeotropic to planar alignment of the nematic liquid crystal after strong rubbing. In order to study the transition, the polyimide surface was investigated by atomic force microscopy, surface free energy measurement and angle-resolved analysis X-ray photo-electron spectroscopy before and after rubbing with a velvet fabric. It was found that both the change of surface polarity and surface morphology were not the reasons for the transition. The droop of the side chain on the polyimide surface after the rubbing treatment was detected by angle-resolved analysis X-ray photo-electron spectroscopy. Owing to the special structure of the novel polyimide, the X-ray photo-electron spectroscopy was successfully used for the first time to analyse the conformational change of the side chain of a polymer. In conclusion, the transition of nematic liquid crystal alignment from homeotropic to planar after rubbing was influenced by the side chain conformation of the polyimide.
Acknowledgements
We would like to acknowledge the useful advice from Makoto Takeishi of Yamagata University. We would also like to thank Hong Cheng, Analytical and Testing Centre of Sichuan University, for her kind assistance in the XPS measurements. This work was supported by the National Natural Science Foundation of China (Grant No. 50973067), Fund from the State Key Laboratory of Polymer Materials Engineering (Sichuan University) and the project sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and Science (No. 20071108-18-12).